On June 1, Kareem sends Fatima an e-mail offering to build her a new garage for $20,000. In his e-mail, Kareem wrote, "acceptance by certified mail is advisable." On June 2 at 8 a.m., Kareem sends Fatima a certified letter attempting to revoke the offer. At 2 p.m. the same day, Fatima mails Kareem a letter via certified mail attempting to accept his offer. Under these circumstances, _____.